4.4-stable review patch. If anyone has any objections, please let me know.
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From: Juergen Gross jgross@suse.com
commit 2ac2a7d4d9ff4e01e36f9c3d116582f6f655ab47 upstream.
In the following situation a vcpu waiting for a lock might not be woken up from xen_poll_irq():
CPU 1: CPU 2: CPU 3: takes a spinlock tries to get lock -> xen_qlock_wait() frees the lock -> xen_qlock_kick(cpu2) -> xen_clear_irq_pending()
takes lock again tries to get lock -> *lock = _Q_SLOW_VAL -> *lock == _Q_SLOW_VAL ? -> xen_poll_irq() frees the lock -> xen_qlock_kick(cpu3)
And cpu 2 will sleep forever.
This can be avoided easily by modifying xen_qlock_wait() to call xen_poll_irq() only if the related irq was not pending and to call xen_clear_irq_pending() only if it was pending.
Cc: stable@vger.kernel.org Cc: Waiman.Long@hp.com Cc: peterz@infradead.org Signed-off-by: Juergen Gross jgross@suse.com Reviewed-by: Jan Beulich jbeulich@suse.com Signed-off-by: Juergen Gross jgross@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/xen/spinlock.c | 15 +++++---------- 1 file changed, 5 insertions(+), 10 deletions(-)
--- a/arch/x86/xen/spinlock.c +++ b/arch/x86/xen/spinlock.c @@ -47,17 +47,12 @@ static void xen_qlock_wait(u8 *byte, u8 if (irq == -1) return;
- /* clear pending */ - xen_clear_irq_pending(irq); - barrier(); + /* If irq pending already clear it and return. */ + if (xen_test_irq_pending(irq)) { + xen_clear_irq_pending(irq); + return; + }
- /* - * We check the byte value after clearing pending IRQ to make sure - * that we won't miss a wakeup event because of the clearing. - * - * The sync_clear_bit() call in xen_clear_irq_pending() is atomic. - * So it is effectively a memory barrier for x86. - */ if (READ_ONCE(*byte) != val) return;